The present invention relates to systems for assembling motor-vehicle body structures or sub-assemblies thereof of the type comprising:    a welding-assembly station;    a conveying line, for carrying at least part of the structures to be assembled to the assembly station and for carrying the assembled structures out of the assembly station;
a pair of frames equipped with locating and clamping fixtures, which can be positioned in the welding-assembly station, at the two sides of the line, for locating in position and clamping part of the body to be assembled;
welding means, for assembling the structure located in position and clamped in the welding-assembly station; and
one or more manipulating robots, for transferring each of said locating and clamping frames between an operative position in the assembly station and an inoperative position, which is remote with respect to the assembly station.
Systems having the characteristics specified above are, for example, described in the documents Nos. WO-A-9512515 and WO-A-9532886. Said systems are distinguished from other known assembling systems substantially in that the locating and clamping frames used for enabling tack-welding of the body are moved by means of robots and are also for this reason called “robotized framers”.
In the robotized framing systems of the type specified above, the motor-vehicle body is tack-welded in the assembly (or “framing”) station, by joining the floor panel of the body, which is fed into the assembly station by the conveying line, to the two sides of the body, which are carried by the aforesaid side locating and clamping frames. For instance, the sides can be prepared on the frames when the latter are in their inoperative position, remote with respect to the welding station. The sides are loaded on the frames by the aforesaid manipulating robots that pick them up from conveyors adjacent to the line. The parts of each side are located in position and clamped by means of the locating and clamping fixtures, with which each frame is provided. When a side is ready on the respective supporting frame, the latter is carried into its operative position in the welding station by means of a manipulating robot. For this purpose, the supporting frame is provided with means for coupling to the wrist of the manipulating robot so that in said step the supporting frame functions as gripping tool (or “gripper”), which the manipulating robot uses for handling the respective sides of the body. When the sides reach the operative position in the welding station, they are assembled with the chassis that is positioned therein, by means of further locating and clamping fixtures, which can be carried by the fixed structure of the welding station and/or by the locating and clamping frames. Also positioned in the same framing station are further transverse elements, which connect the two sides of the motor-vehicle at the top in order to impart greater stability on the structure. The structure thus positioned and clamped is subjected to a series of assembly operations, typically welding operations, for example, electrical spot welding and/or laser welding. Typically, the welding fixtures are carried by manipulating robots, which can be at least in part the same robots used for handling the frames, which, once the frames are positioned in their operative position, are released from said frames and equip themselves with welding fixtures for executing the necessary welding operations.
Once welding is completed, the clamping fixtures of the positioning frames are brought into an open condition, disengaged from the welded structure, so that the latter is free to be carried out of the welding station by means of the conveying line.
Systems of this sort are also flexible; i.e., they are also able to operate on different types or models of body, fed along one and the same line. Different models of body imply a different conformation and/or positioning of the locating and clamping fixtures prearranged on the side frames. For this purpose, for each model of body there is provided a corresponding pair of frames with the necessary locating and clamping fixtures. The manipulating robots position in the assembly or framing station the pair of supporting frames corresponding to the type of body that each time is to be assembled in the welding station. For this purpose, pre-arranged adjacent to the welding station are stationary magazines for different types of frames corresponding to different types of body, and, whenever necessary, the manipulating robots put a frame of a type previously used back into a magazine and pick up from another magazine a new type of supporting frame corresponding to a new type of body that must be assembled in the framing station.
A drawback of the systems that have been developed so far lies in the fact that they require pre-arrangement of a structure for locating and clamping of the body to be assembled that is sufficiently rigid to guarantee the necessary quality in obtaining an assembled structure having the pre-set dimensional characteristics. On account of the tolerances of fabrication, it is possible in fact for the body that is formed in the welding station to present, for example, a width greater than the nominal dimension, with the consequence that, when it is located and clamped in the welding station, it exerts a force on the side locating and clamping frames that tends to move the latter away from one another. The precision of the geometry of the welded body is determined, on the one hand, by the fact that the components carried by each frame are located precisely in position with respect to the frame, and on the other by the fact that each frame is located with precision in position when it is in its operative position in the welding station. If said position is not guaranteed precisely, for example, in so far as the structure of the frames is not sufficiently rigid, there follows a lack of dimensional quality of the structures obtained. For this reason, in known systems, the side frames must be produced with a relatively cumbersome and heavy structure and/or are provided with projecting cross members that connect up to one another in the operative position of the two side frames to provide a rigid cage (see, for example, the document No. WO-A-9964289), which involves a further increase in weight, as well as certain additional constraints for the designer, who must avoid the interference between said cross members and the parts of the sides of the body carried by the supporting frames. In other cases (WO-A-9532886), there is envisaged the construction of an entire cage surrounding the body on a number of sides, with the consequence that the side supporting frames must present a structure that is even more cumbersome and heavier. A greater weight and a greater encumbrance of the frames also involves greater difficulties of movement by means of the manipulating robots, which must consequently be sized for the loads to be handled, with an increase in the final cost of the system.